The present invention relates to digital to analog converters (DAC), and more particularly, to a method for compensating gain mismatch between two DACs, and an apparatus thereof.
Generally speaking, no matter what kind of modulation scheme is employed by a communication system, DACs to convert digital modulated signals into analog signals. Analog signals are then processed, for example, amplified or frequency upgraded, to generate final analog signals for transmission. When designing a communication system, DACs are usually assumed to be ideal components. Unfortunately, DACs employed by a transmitter have some less than ideal characteristics. One non-ideal characteristic is referred to as “IQ gain mismatch effect”. More specifically, in a transmitter, the gain of a DAC in an in-phase channel (I channel) is usually slightly different from the gain of another DAC utilized in a quadrature channel (Q channel). The IQ gain mismatch effect deteriorates signal quality. Hence, for communication systems requiring high signal transmission quality, the IQ gain mismatch effect is a problem that demands careful handling.
Generally speaking, two compensation schemes can be utilized to solve the IQ gain mismatch problem. The first is a digital compensation scheme. The second is an analog compensation scheme. The digital compensation scheme operates in the digital domain (i.e. digital signals are compensated before being applied to DACS). The analog compensation scheme operates in the analog domain (i.e. analog signals are compensated after being generated by DACS). Since the digital compensation scheme requires a great amount of computation, the analog compensation scheme is always utilized by communication systems.
To compensate a gain mismatch between two DACs of a transceiver's I transmission channel and Q transmission channel, a conventional method utilizes analog to digital converters (ADCs), which already exist in the transceiver, as measuring tools for determining the gain mismatch. FIG. 1, FIG. 2, and FIG. 3 are diagrams illustrating how devices are arranged when the method of the related art is utilized to compensate for a gain mismatch between two DACS. First, in FIG. 1, a DAC 110 in an I transmission channel of a transceiver is connected to an ADC 130 in an I receiving channel of the transceiver; a DAC 120 in a Q transmission channel of the transceiver is connected to an ADC 140 in a Q receiving channel of the transceiver. Two digital values ZI and ZQ received by a computation module 150 are as follows:ZI=ρZYI·ρYXI·XI  (1)ZQ=ρZYQ·ρYXQ·XQ  (2)                where ρYXI and ρYXQ are gain values of the DACs 110 and 120 respectively; Σzyi and ρZYQ are gain values of the ADCs 130 and 140 respectively.        If XI is equal to XQ, after divides ZI by ZQ, then the computation module 150 can get the following equation:        
                                                                        R                1                            =                              (                                                      Z                    I                                    /                                      Z                    Q                                                  )                                                                                        =                                                (                                                            ρ                      ZYI                                        /                                          ρ                      ZYQ                                                        )                                ·                                  (                                                            ρ                      YXI                                        /                                          ρ                      YXQ                                                        )                                ·                                  (                                                            X                      I                                        /                                          X                      Q                                                        )                                                                                                        =                                                G                  RX                                ·                                  G                  TX                                ·                1                                                                        (        3        )            
where GRX=ρZYI/ρZYQ represents a gain mismatch between the ADCs 130 and 140; GTX=ρYXI/ρYXQ represents a gain mismatch between the DACs 110 and 120.
Next, in FIG. 2, the DAC 110 in the I transmission channel is connected to the ADC 140 in the Q receiving channel; the DAC 120 in the Q transmission channel is connected to the ADC 130 in the I receiving channel. The digital values ZI and ZQ received by the computation module 150 are as follows:ZI=ρZYI·ρYXQ·XQ  (4)ZQ=ρZYQ·ρYXI·XI  (5)
If XI is equal to XQ, after divides ZI by ZQ, the computation module 150 can get the following equation:
                                                                        R                2                            =                              (                                                      Z                    I                                    /                                      Z                    Q                                                  )                                                                                        =                                                (                                                            ρ                      ZYI                                        /                                          ρ                      ZYQ                                                        )                                ·                                  (                                                            ρ                      YXQ                                        /                                          ρ                      YXI                                                        )                                ·                                  (                                                            X                      Q                                        /                                          X                      I                                                        )                                                                                                        =                                                G                  RX                                /                                  G                  TX                                                                                        (        6        )            
With equation (3) and equation (6), it is quite a simple job for the computation module 150 to determine the gain mismatch values GTX and GRX. More specifically, these two values can be determined according to the following equations:GTX=√{square root over (R1/R2)}  (7)GRX=√{square root over (R1●R2)}  (8)
After the gain mismatch value GTX between the I channel DAC 110 and the Q channel DAC 120 is determined, under a normal operation mode, the related art system arranges the transmitting part of the transceiver into the configuration shown in FIG. 3. More specifically, the output end of the Q channel DAC 120 is connected to a gain amplifier 160. Certainly, the gain amplifier 160 can also be connected to the output end of the I channel DAC 110 rather than the Q channel DAC 120. With the gain amplifier 160, an analog signal YQ generated by the DAC 120 is amplified into another analog signal Y′Q, the gain mismatch between the DAC 110 and DAC 120 is then compensated. In other words, the gain of the DAC 110 is equal to the gain of the DAC 120 multiplied with the gain of the gain amplifier 160. However, since the gain of the gain amplifier 160 is determined by a control value C, the computation module 150 of the related art must be built with a mapping relationship between the gain of the gain amplifier 160 and the control value C. With the built-in mapping relationship, the computation module 150 can then determine a suitable calibration control value CC according to the determined gain mismatch value GTX.
The method of the related art must survey many sample chips to determine an averaged mapping relationship then set the averaged mapping relationship in each individual system utilizing the chips. However, gain amplifiers on different chips do not always have the same characteristics. Even within a single chip, characteristic parameters of the chip do not always remain fixed. Hence, the method of the related art does not always compensate for the gain mismatch effect of two DACs accurately and is therefore not an optimal solution.